Biological and synthetic tissue adhesives have been developed as alternatives to sutures and staples for adhering biological tissue. Examples of biological tissue adhesives include fibrin glues. Examples of synthetic tissue adhesives include cyanoacrylates, urethane prepolymers, and gelatin-resorcinol-formaldehyde. Applications of adhesives to biological tissue range from soft (connective) tissue adhesion to hard (calcified) tissue adhesion. Soft tissue adhesive are used both externally and internally for wound closure and sealing. Hard tissue adhesive adhesives are used in applications that include bonding prosthetic materials to teeth and bone.